Field of the Invention 1. This invention relates to sensors for detecting fluid levels within a container, and more specifically to fluid level sensors for use in fuel tanks containing gasoline, diesel, or other volatile fuels.
The most commonly used fluid level sensor is the variable resistor sensor utilizing a float to produce a resistance change in the variable resistor. As the float moves vertically with the fluid level, the electrical resistance of the sensor changes typically from 30 to 270 ohms. In most sensors, a sliding or moving contact attached to the float establishes a resistive circuit based upon the position of the contact with respect to a wirewound resistor or a thick film resistor printed on an insulating base or substrate.
Other approaches to fluid level detection include use of resistors with large temperature coefficients, known as thermistors, located at various vertical positions in the fluid reservoir. As electrical power is applied to the resistors, the devices immersed in the fluid remain cool while those that are exposed to air will increase in temperature and produce a change in overall resistance of the device. Extensive signal conditioning and temperature compensation circuitry is typically required with such a sensor to create a usable signal. Fluid compatibility and manufacturing costs limit widespread acceptance of this type of device.
A vertical sensor with a sliding contact has been used in some automotive applications. Typically a float provides a contact point with respect to a resistor. The resistor is usually a wire helix wound about an insulating mandrel.
Examples of prior art fluid level sensors are shown in the following patents: U.S. Pat. Nos. 4,637,254 to Dyben et al., 4,779,460 to Cruickshank, 3,113,282 to Coleman, 3,106,693 to de Giers, 2,484,690 to de Giers, and Italian Patent No. 619,958 to Carlo Ceresa et al.
An example of thick film resistor technology used in a liquid level sensor is shown in Weaver, U.S. Pat. No. 4,920,798. The Weaver device includes a thick film resistive coated plate with a slidable contact member providing a resistive signal in proportion to the position of the float.
Trucks and large vehicles which have fuel tanks with a depth of 50 centimeters or more have experienced long-term reliability problems with fuel sensors. In addition, accuracy of fluid level detection is also a problem. Further, vehicle design and space requirements may restrict the mounting location for the sensor to the side or bottom of the fuel tank as a result of interferences. Leaks attributable to the mounting location and resistor failure due to vibration are common occurrences in such applications. Many different physical sizes and operating geometries are required due to the significant number of different tank and vehicle mechanical designs. Typical tank depths for trucks may range from 30 centimeters to 150 centimeters.
Off-road vehicles such as bulldozers, cranes, loaders and forklifts often are not equipped with fuel level sensors. Typically such vehicles are produced in small quantities and the economics of producing sensors for these applications are cost prohibitive in view thereof. The anticipated long service life and extreme vibration of physical movement of this type of equipment renders conventional sensors impractical. Tank depths in this field vary from 60 centimeters to 200 centimeters. Since many of these tanks are long and narrow, the sensor may be subjected to significant wear as a result of constant fuel sloshing within the tank. Storage tanks used for petroleum products and other liquids often use a mechanical or manually operated dipstick for measuring fluid level within the tank. Electronic sensors and indicators have not generally been cost effective in this particular application. Tank materials may be metal or plastic or combinations thereof, and tank depths may extend up to 4 meters.
A liquid level sensor for use in harsh environments suitable for larger liquid containers is needed to resolve the above difficulties which arise in detecting fuel level within a container.